FAST v8 AeroDyn v15 convergence problems for low-speed, high-turbulence

Dear all,

I using FAST v8 with AeroDyn v15 to simulate the NREL 5MW turbine (with Sandia 5MW blade definition) in TurbSim-generated turbulence fields (normal and extreme turbulence) throughout the operating range. Simulation setup is based on Certification Test #18. I am using the steady airfoil aerodynamics model (AFAeroMod = 1), an uncoupled skewed wake correction model (SkewMod = 1), and I do not include tower aerodynamic effects.

For some wind conditions (extreme turbulence model at 3, 5, 7, and 9 m/s and normal turbulence model at 3 m/s), the BEM solution is failing well into the simulation (anywhere from 100 s to 1200 s in) with a negative Vx. For example:

[code] FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 7, blade
3):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating
conditions! Vx = -1.50015E-02, Vy = 14.886, rlocal = 19.936, theta = 0.17736

FAST encountered an error at simulation time 154.6 of 200 seconds.
Simulation error level: FATAL ERROR[/code]
Although I wouldn’t expect these steps to be helpful in this case, I have tried adding a correction step, making DT smaller, and changing InterpOrder, all to no avail. I set the initial conditions according to the discussion here [url]http://forums.nrel.gov/t/fast-errors-for-high-speed-turbulence-cases/1509/1] (that post had to do with simulations failing right away, which was solved by setting correct initial conditions–now the failure happens well into simulation). I understand that Vx is the relative wind speed (wind speed minus structural speed); the structural solution seems reasonable up until failure for my simulations.

Based on the discussion with Yannick in [url]http://forums.nrel.gov/t/aerodyn-v15-convergence-issues/1305/7] it appears that problems stemming from small/negative Vx may be a known issue. Is that the case? Is there anything else I might try? Otherwise I may try to see if there is any difference using AeroDyn v14.

Thank you,
Austin Herrema

Dear Austin,

The failure of the BEM algorithm of AeroDyn v15.00 - v15.03 when Vx goes negative is a known issue. Working in collaboration with Envision, we have found the algorithmic problem causing the issue. While we have not yet publicly released the updated code, you can find a version of AeroDyn v15 with issue resolved here: github.com/NWTC/AeroDyn. Reverting to the older AeroDyn v14 will also work.

Bets regards,

Dear Dr. Jonkman,

Thank you for the response; it is good to know that this issue is already being addressed and that a solution has been made available. Thanks to the NREL team (and Envision) for their great work!

Kind regards,
Austin

Dear Dr. Jonkman,

I’m using FAST8.16-Beamdyn, Aerodyn15 to simulate my case. it works well when I set to CompElast=1,CompAer=2.
when I switch to CompElast=2, it appears an error like in snapshot.
I checked the Beamdyn Distributed properties and blade geometry files, it ran in Beamdyn Stand alone without any error.
Moreover I tried to change ElemOrder=8, and DT=0.000125.

Kind regards,
Tohid.

Dear Tohid,

You asked a similar question in the following forum topic: http://forums.nrel.gov/t/creating-beamdyn-blade-input/1325/16. I’ve responded there. Please don’t post the same question in multiple forum topics.

Best regards,

Dear Jason,

i appologize for the mistake. I will follow my question in aforementioned link.

Best regards,
Tohid.

Dear Jason,

I tried to simulate a full nacelle rotation with still wind using AeroDyn 15 and the yaw maneuver. As I know from the posts in this section the error I got, is a well known problem. So I tried to include the release of AeroDyn, you posted the link for, but unfotunatly I still get the error with the new compiled FAST version. My wind field is uniform with a constant speed of 0.5 m/s in horizontal direction and no shear.

I guess, I did not include the source code for the changed AeroDyn in a correct way.
Do you have a short manual how to include the changed AeroDyn into the FAST source code, especially concerning the correct libary NWTC?

Thank you in advance.
Best regards,
Fiona

Dear Fiona,

Compiling FAST v8.16 with AeroDyn v15.04 has been discussed in the following forum topic: http://forums.nrel.gov/t/compile-aerodyn-15-04-source-files-in-fast/1602/8.

Best regards,